The following references are illustrative of the present state of the art in time-division telephone exchanges:
1. The Japanese literature entitled "H-type Automatic Telephone Switching System", pp. 157-158, chap. 4.1.3;
2. The Japanese Patent Publication No. 16971/66;
3. Bell Laboratories Record, Vol. 51, No. 8, pp. 226-232, September 1973 entitled "No. 4 ESS-Long Distance Switching for the Future" written by Mr. G. Douglas Johnson; and
4. International Switching Symposium 74, pp. 212/1-212/5 entitled "An Experimental Digital Local System" written by Mr. H. S. McDonald.
The present invention relates generally to time-division telephone exchanges and, more particularly, to an immediate ring-back control system for a time-division telephone exchange, which first sends a continuous ring-back tone to a calling subscriber for a given time period and then sends a periodically interrupted ring-back tone at the time of connection of the ring-back tone to the calling subscriber in order to allow the calling subscriber to receive the ring-back tone immediately after the calling subscriber has dialled a called telephone number.
It is desirable to shorten the period of time required when a calling subscriber has dialled a telephone number, a speech path to a called subscriber has been established and the ring-back tone is received by the calling subscriber, that is, to reduce post dialling delay. The post dialling delay includes, in addition to the period of time for establishing the speech path, a delay time from the beginning of ready state of calling the called subscriber in a terminating office to the time of the actual transmission of the ring-back tone to the calling subscriber. This delay time has a significant influence on the telephone service. For example, in the case where the ring-back tone is intermittently sent to the calling subscriber with a one-second mark and a two-second space, there exists an instance where a maximum of two-seconds delay time is required before the ring-back tone is sent to the calling subscriber.
In order to reduce such a delay time to improve the serviceability, a first ringing control system has been heretofore used, in which a continuous ringing signal is sent to the called subscriber once for a predetermined period of time after the establishment of the speech path and the continuous ring-back tone is sent to the calling subscriber, and thereafter a periodically interrupted ring-back signal and ring-back tone are sent. However, in such a case, since this prior art first ringing control system requires a first ringing control circuit for each of the ring-back tone trunk circuits, it has a drawback in that the cost of the ring-back tone trunk circuits is high.